Electrical conductivity anisotropy in dry and hydrous olivine at 8 GPA

The effects of dissolved H2O on the electrical conductivity and its anisotropy in olivine (Fo90) at 8 GPa were investigated by complex impedance spectroscopy. At nominally anhydrous conditions, conduction along [1 0 0] and [0 0 1] is slightly higher than along [0 1 0] in contrast to observations made at lower pressures in earlier studies. Increasing H2O content increases conductivities but activation energies are lower and H2O concentration dependent. The use of polarized FTIR spectroscopy to determine H2O concentrations reveals a weaker than expected effect that water has on olivine conductivity and distinguishes our results from earlier studies based on analyses using non-polarized infrared spectroscopy. We show that at H2O concentrations of a few hundred wt ppm or less, that the dominant conduction mechanism at mantle temperatures continues via small polarons, such as that observed for anhydrous olivine. Our results also suggest that at depths greater than 200 km, the presence of H2O may not be necessary to explain regions in the upper mantle where both electrical and seismic anisotropy are observed. This can be explained by differences in the pressure dependence of the activation energy for conduction along each of the three crystallographic axes. However, while electrical anisotropy of anhydrous olivine remains weak at 8 GPa, it is nevertheless enhanced by elevated concentrations (>several hundred wt ppm) of dissolved H2O. At these conditions dominated by proton hopping, conductivity along [0 1 0] is highest, approximately an order of magnitude greater than along [1 0 0]. Additionally, at 1000 wt ppm and 1500 ° C, an isotropic conductivity derived from the data is about 1 order of magnitude higher than that for nominally anhydrous olivine. Thus, in regions of the mantle characterized by anomalously high conductivities and both electrical and seismic anisotropy, significant amounts of dissolved hydrogen can be expected.